1. Field of the Invention
The present invention relates to a production method of an active matrix substrate, an active matrix substrate, and a liquid crystal display device. More specifically, the present invention relates to a production method of an active matrix substrate in which an interlayer insulating film effective for improvement in aperture ratio is formed, an active matrix substrate, and a liquid crystal display device obtained by using the same.
2. Description of the Related Art
Active matrix substrates have been widely used in active matrix display devices such as a liquid crystal display device and an EL (Electroluminescence) display device. In an active matrix substrate used in a conventional active matrix liquid crystal display device, a switching element such as a TFT (Thin Film Transistor) 52 is disposed at each intersection of a plurality of scanning signal lines 53 with a plurality of data signal lines 54, as shown in FIG. 13, and by its switching function, a pixel signal is transmitted to each pixel (variable capacitance) 50 and a storage capacitance 55.
With respect to a configuration of an active matrix substrate used in a conventional active matrix liquid crystal display device, a configuration in which an interlayer insulating film formed by an organic film with high transparency is formed above a TFT, a scanning signal line, and a data signal line, and over the interlayer insulating film, a pixel electrode made of a transparent conductive material is formed, is known (for example, refer to Japanese Kokai Publication No. Hei-09-152625 1). In such a configuration, with respect to an electrical connection between the pixel electrode with a drain electrode of the TFT, (1) the pixel electrode, (2) a contact hole formed in the interlayer insulating film on a pattern of a storage capacitance (common) wiring or the scanning signal line, (3) a storage capacitance upper electrode, (4) a drain leading wiring, (5) a drain electrode of the TFT, are connected and conducted to each other in this order. In such an active matrix substrate including the pixel electrode formed on the interlayer insulating film, the pixel electrode can be formed to overlap each signal line, and therefore the aperture ratio can be increased and an effect of shielding an electric field from each signal line to the pixel electrode can be obtained.
In production steps of such an active matrix substrate, static electricity is generated between electrodes or between wirings formed on a supporting substrate due to friction when the substrate is transported, a plasma treatment in dry etching or ashing, a shower of an etching solution or a cleaning solution to the substrate, and the like, and thereby charging is generated. An insulating substrate is generally used as a support substrate of the active matrix substrate. Therefore, a difference in electric potential, generated by the induced static electricity, is poorly eliminated, and when the difference in electric potential between electrodes or between wirings exceeds a dielectric strength voltage of an insulating film, electric discharge occurs. As a result, electrostatic discharge (ESD) (also, referred to as electrostatic breakdown) of the insulating film, heat generation of the semiconductor, and the like, may be generated. If they occur, unrecoverable permanent breakdown, partially recoverable semi-permanent breakdown, characteristic degradation due to variation of a threshold voltage or reduction in mobility, and further reduction in long-term reliability due to potential defects, may be generated in TFTs.
A method of forming a wiring for short-circuit (also referred to as short-circuit wiring) for short-circuiting a plurality of scanning signal lines or data signal lines, thereby preventing generation of differences in electric potential between electrodes or wirings was disclosed as a conventional method of preventing such electrostatic discharge (for example, refer to Japanese Kokai Publication No. Hei-11-15017, Japanese Kokai Publication No. 2000-235195, Japanese Kokai Publication No. Hei-11-109416, and Japanese Kokai Publication No. Hei-09-61850). For example, in Japanese Kokai Publication No. Hei-11-15017, Japanese Kokai Publication No. 2000-235195, and Japanese Kokai Publication No. Hei-11-109416, a ring-shaped conductor pattern (short ring) is formed around a display region where pixel electrodes are disposed in a matrix pattern. Such a wiring for short-circuit such as a short ring is separated from electrodes or wirings by laser irradiation, separation of the substrate, and the like, in a step after completion of an active matrix substrate. As a result, the short-circuit between the electrodes or the wirings can be eliminated, and thereby the active matrix substrate can function normally.
However, such a conventional method of forming the wiring for short-circuit has room for improvement in that a difference in electric potential between a source electrode and a drain electrode of a TFT disposed in each pixel can not be eliminated although a difference in electric potential between scanning signal lines or between data signal lines, or between a scanning signal line and a data signal line, can be eliminated by forming the wiring for short-circuit. In the production steps of the active matrix substrate, the source electrode and the drain electrode are generally formed by separating a low resistance semiconductor layer by channel etching. In this case, the source electrode is connected to the wiring for short-circuit through the data signal line, but the drain electrode is connected to only the data signal line (or the source electrode) through a high resistance semiconductor layer. Therefore, the drain electrode is electrically isolated, and the electrical isolation is not eliminated even after a contact hole or a pixel electrode is formed in a step performed later. Therefore, charging easily occurs, and difference in electric potential between the drain electrode and the source electrode tends to be generated. As a result, if this difference in electric potential exceeds a dielectric strength voltage at a TFT channel, electrostatic discharge at the TFT channel occurs.
A method of forming a wiring for short-circuit for conducting a source electrode to a drain electrode was disclosed as a conventional method of eliminating such a difference in electric potential generated between the source electrode and the drain electrode of the TFT disposed in each pixel (for example, refer to Japanese Kokai Publication No. Hei-08-114815, Japanese Kokai Publication No. 2001-133807, and Japanese Kokai Publication No. 2001-255557).
However, the above-mentioned method in which during preparation of the TFTs, the wiring for short-circuit for temporarily conducting the source electrode to the drain electrode is formed to prevent the electrical isolation of the drain electrode from the source electrode has room for improvement in that an additional step of separating the wiring for short-circuit is needed. Japanese Kokai Publication No. Hei-08-114815, Japanese Kokai Publication No. 2001-133807, and Japanese Kokai Publication No. 2001-255557 fail to disclose a method of disposing the wiring for short-circuit and a position where the wiring for short-circuit is separated without reduction in transmittance when the wiring for short-circuit is disposed in a pixel, and have room for improvement also in that an aperture ratio of the pixel is reduced when the wiring for short-circuit is disposed in the pixel. Further, in Japanese Kokai Publication No. 2001-133807, the wiring for short-circuit is made of polysilicon, and therefore has a resistance about 100 times larger than that of a low resistance metal material such as aluminum and molybdenum generally used for forming the data signal line. Therefore, there is room for improvement in that such a wiring for short-circuit is difficult to sufficiently function.